The ATmega8U2 firmware source code is available . The ATmega8U2 is loaded with a DFU bootloader, which can be activated by connecting the solder jumper on the back of the board and then resetting the 8U2. You can then use Atmel's FLIP software (Windows) or the DFU programmer (Mac OS X and Linux) to load a new firmware. Or you can use the ISP header with an external programmer (overwriting the DFU bootloader). See this user-contributed tutorial for more information.

There is no jumper on the back of the board and the link only goes to a page that new users can not access.

It looks like http://arduino.cc/forum/index.php?topic=111.0 is the new location of the tutorial. I tried to edit the page to correct that, but don't have privileges and can't find contact details for any of the previous editors (Scott Fitzgerald / Federico).

From the schematic for the USB2Serial Light, HWB comes out on pin 6 of the flat header opposite the USB, so can be connected to pin 1 on the same header to pull it down into reprogramming mode.

In my case even after going through a reset with HWB pulled down, the board still reports as VID 0000 and PID 0000 so I guess it is faulty, but hope that helps you/others looking for the tutorial from the broken link on http://arduino.cc/en/Main/USBSerial